This invention relates to power-driven conveyors and, more particularly, to low-friction conveyors using modular conveyor belts. The invention also relates to methods for making such conveyor belts.
When articles being conveyed on a moving conveyor start to back up, trailing articles push against leading articles. The result is a buildup of backline pressure, which is greatest on the lead articles. Too much backline pressure can crush or otherwise damage the articles and load the conveyor because of the dynamic friction between the moving conveyor and the stalled or slowly moving articles.
In the corrugated industry, stacks of corrugated sheets are conveyed along a processing line. A common way to convey these stacks is with powered roller conveyors. In these conveyors, parallel cylindrical rollers with axes of rotation transverse to the conveying direction are arranged to form a rolling conveyor bed. Drive belts are often used to contact the rollers to rotate them and propel the stacks along the roller bed. To eliminate backline pressure by preventing consecutive stacks from bumping into each other, the roller conveyor is divided into successive accumulation zones of fixed length The rollers in one zone are powered independently of those in another zone. In this way, when a downstream stack is stopped in one zone of the conveyor, the trailing upstream stack can be moved from zone to zone and stopped in the zone just upstream of the stopped downstream stack without contact. Various drive arrangements are used to achieve individual zonal control by selectively engaging the rollers in each zone with the drive belt.
In another version, a conveyor belt is flanked on each side by a roller conveyor bed. The stack of corrugated sheets rests atop both roller conveyor belts. Portions of the conveyor belt are raised and lowered into and out of contact with the bottom of the stacks. When raised into contact, the conveyor belt transports the stack along; when the belt is lowered out of contact, the stack rests in place on the two roller conveyor beds. Thus, each portion of the conveyor belt that can be raised and lowered defines an accumulation zone.
But these zero-back-pressure roller conveyors have shortcomings. The rollers have a tendency to freeze up or their mounting holes to wallow out over time, resulting in such performance deficiencies as increased friction against the conveyed stacks, a bumpy conveyor bed, and excessive noise. Roller conveyors also cause a stack of corrugated to form an “elephant foot” as it is conveyed. There are a couple of causes for the “elephant foot.” As the stack traverses the spacing between consecutive rollers, the leading edge of the bottom-most sheets bumps into the upcoming roller. Each time this occurs, the sheets above tend to creep forward relative to the bottom sheets. Article creep is also caused by a wave effect. The weight of the stack on the bottom-most sheets makes them conform to the rollers. The closer a sheet is to the bottom of the stack, the more it deforms around the rollers into the inter-roller gaps and adopts a wavy shape. As the stack moves over the rollers, the wave dynamically propagates upward into the stack, causing adjacent sheets at the bottom of the stack to creep. On a long conveying path over many rollers, the side profile of the stack resembles an “elephant foot” with the leading edge of the bottom-most sheet lagging the leading edge of the topmost sheets. If the “elephant foot” becomes too exaggerated, the stack becomes unstable, and sheets tip over, requiring manual intervention to rearrange the stack.
One way to achieve zero back pressure and minimize the “elephant foot” problem is to use a series of conveyor belts, or chains, arranged end to end with a small space between consecutive belts. Each belt, which forms an accumulation zone, is individually controlled by its own drive train and sprockets or pulleys. The flat conveying surfaces provided by the belts avoid the bumpiness of a roller conveyor, and the “elephant foot” problem is minimized. But such an arrangement is more complex and costly in that multiple sprockets, shafts, and drive motors are required to handle all of the zones, especially in a long conveyor system. Furthermore, all these fixed-length zone systems cannot efficiently accumulate stacks of different sizes or stacks that take up more conveyor space than the length of a zone or much less space then the length of a zone.
Modular conveyor belts, especially modular plastic conveyor belts, are widely used in the food processing industry to convey food and beverage products. These endless belts are generally looped around sets of drive sprockets or drums at opposite ends of a conveyor section. The products are carried along the carryway portion of the belt's path, and the belt returns below along a returnway. The belt is supported in the carryway, which can be a solid plate or wearstrips. The belt slides along the carryway as it is driven. In most food-handling applications, these belts are relatively lightly loaded, and the friction between the belt and the carryway is not too great a problem
But, in transporting heavy loads, such as stacks of corrugated sheets, the friction between a belt and its carryway can be significant, requiring larger drive motors and resulting in accelerated belt wear and jerky belt motion. These are some of the reasons that roller conveyors are usually used in the corrugated industry. And, in some applications, plant space is limited. Larger-diameter drive sprockets may not be possible because of vertical space limitations on the spacing between the belt's carryway and returnway. Big drive motors may also be unable to meet the space requirements. Furthermore, limitations on motor size limit conveyor length.
Thus, there is a need for a low-friction conveyor capable of compactly transporting heavy loads and avoiding the problems caused by roller conveyors.